Girlfriend has stomach pain after anal?

I'd never really given anal two thoughts before she asked about it last year. When she talked about wanting to try it, the more I started to become obsessed with it. We finally tried it at the very end of the year and have done it three or four times since. We both loved every second and totally enjoyed it, and I've wanted it many, many more times but it hasn't really happened. She finally confessed a couple weeks ago that it's hard for her because she knows I crave it and she loves the way it feels but for two or three days afterward she has bad stomachaches (but her ass feels just fine) and she can't go to the bathroom.

After a little knocking around on the net it sounds like it's indigestion/discomfort from air being pushed up inside her and into her stomach, but that certainly makes it hard to feel anal is worth it. Anyone have advice on how to prevent this?